(1) Field of the Invention
The present invention generally relates to a malfunction detecting apparatus of an evaporated fuel purge system, and more particularly to an apparatus for detecting a malfunction in an evaporated fuel purge system in which evaporated fuel from a fuel tank is absorbed in a canister and the evaporated fuel of the canister is supplied to an intake passage of an engine via a purge line when the engine is operating under a prescribed operating condition.
(2) Description of the Related Art
An internal combustion engine of an automotive vehicle is provided with an evaporated fuel purge system. In this evaporated fuel purge system, evaporated fuel supplied from a fuel tank is absorbed by an absorbent of a canister, and the evaporated fuel of the canister is supplied (or purged) to an intake passage of the engine when the engine is operating under a prescribed operating condition. The flow of the evaporated fuel from the canister to the intake passage is adjusted by means of the evaporated fuel purge system, so as to suitably control the flow of an air-fuel mixture from the intake passage to the combustion chamber of the engine. In order to prevent the evaporated fuel from escaping from the system to the atmosphere, the respective parts of the evaporated fuel purge system are hermetically closed.
Occasionally, the evaporated fuel purge system including the fuel tank and the canister may malfunction. For example, a vapor line between the fuel tank and the canister may be damaged, a piping line within the system may be separated, and a purge line between the canister and the intake passage may be clogged. If the vapor line is damaged or the piping line is separated, the evaporated fuel from the fuel tank will escape from the system to the atmosphere, and thus wasted. If the purge line is clogged, the evaporated fuel from the fuel tank will overflow the canister, and thus the evaporated fuel of the canister will leak to the atmosphere via the air inlet opening of the canister.
Therefore, it is necessary to accurately detect the occurrence of any malfunction in the evaporated fuel purge system in order to maintain the exhaust emission and the drivability at an appropriate level.
The inventor of the present invention has proposed several malfunction detecting apparatuses for the evaporated fuel purge system. For example, a proposed malfunction detecting apparatus employs a purge control valve arranged at an intermediate portion of the purge line between the canister and the intake passage, and this purge control valve is switched on and off so as to open and close the purge line. See Japanese Patent Application No. 4-23952. The invention disclosed in this application is assigned to the assignee of the present invention.
In the proposed apparatus mentioned above, the purge control valve is switched on to open the purge line so that an evaporated fuel line of the system is subjected to a negative pressure of the intake passage, and a start pressure of the evaporated fuel pressure line is measured. The purge control valve is switched off after a predetermined time to close the purge line, and an end pressure of the evaporated fuel line is measured. The malfunction discrimination is made based on a pressure change between the measured start pressure and the end pressure.
Another proposed malfunction detecting apparatus uses the purge control valve arranged in the purge line and an internal pressure control valve arranged in the vapor line between the fuel tank and the canister. In this apparatus, the purge control valve is switched on to subject the evaporated fuel line of the system to a negative pressure of the intake passage of the engine. The internal pressure control valve is switched on and off, and a pressure of a first portion between the internal pressure control valve and the fuel tank and a pressure of a second portion between the internal pressure control valve and the canister are measured by a pressure sensor. The malfunction discrimination for the first portion and the malfunction discrimination for the second portion are separately made based on the respective measured pressures. See Japanese Patent Application No. 4-258331. The invention disclosed by this prior application was assigned to the assignee of the present invention.
In the proposed apparatus disclosed in Japanese Patent Application No. 4-23952, a vacuum switching valve (VSV) is used as the purge control valve, and this valve is switched on and off by performing a duty ratio control process. In the evaporated fuel purge system, when a great amount of fuel vapor absorbed in the canister is supplied to the intake passage via the purge line, the air fuel mixture fed from the intake passage to the engine is considerably affected by the amount of the purged fuel vapor supplied from the canister to the intake passage. The exhaust emission performance and the drivability are likely to be deteriorated due to the change of the air fuel mixture on such an occasion. In order to prevent this, it is necessary to suitably adjust the flow rate of the purged fuel vapor from the canister to the intake passage in accordance with the flow rate of the intake air of the engine.
In the proposed apparatus mentioned above, a duty ratio control process for the purge control valve (VSV) is performed in order to suitably adjust the flow rate of the purged fuel vapor. To minimize the deterioration of the exhaust emission, the purge control valve (VSV) is switched on and off by a duty ratio control signal indicating a small duty ratio, which signal is determined by the duty ratio control process.
However, in the evaporated fuel purge system, a fluctuation of the purge line pressure occurs when the duty ratio control process for the VSV is performed in the manner described above. This pressure fluctuation is relatively large when the purge line between the canister and the intake passage is not completely opened by the VSV, and the maximum level of the pressure fluctuation is reached when the duty ratio of the VSV is around 50%.
The duty ratio control process for the purge control valve (VSV) is performed to make the duty ratio of an on-time of the VSV within a duty cycle to a total duty-cycle time as small as possible, as indicated in FIG. 1A, so that the purge line of the evaporated fuel purge system is subjected to a negative pressure of the intake passage for a shorter time. The malfunction discrimination of the system is made based on the measured pressure of the purge line when the duty ratio control process is performed. If the measured pressure is higher than a reference pressure indicated in FIG. 1B, it is determined that a malfunction in the system has occurred. If the measured pressure is lower than the reference pressure, it is determined that the system is normally operating, that is, that no malfunction has occurred.
However, the conventional evaporated fuel purge system has a problem in that the purge line pressure during a malfunction detecting process may fluctuate considerably, as indicated by a zigzag line B in FIG. 1B. The measured pressure output from the pressure sensor is detected as being instantaneously lower than the reference pressure, although the purge line pressure is higher than the reference pressure mostly as indicated by a curve line A in FIG. 1B. Therefore, in the case of the example shown in FIG. 1B, it may be erroneously determined that no malfunction in the system has occurred, even though the evaporated fuel purge system is actually malfunctioning, or the evaporated fuel is leaking in any part of the system.